Myelin basic protein (BP) has been shown to be phosphorylated both in vivo and in vitro by an endogenous protein kinase. We have provided biochemical and immunochemical evidence for the in vivo phosphorylation of four myelin basic proteins in the rat brain. We propose to examine whether BP is first phosphorylated in the glial cell cytoplasm and is then incorporated into the myelin membrane. The developmental studies on in vivo phosphorylation are designed to answer this question. Similarly, determination of the sites of phosphorylation in rat brain myelin BP will elucidate whether the same protein kinase is responsible for the phosphorylation of BP both in vivo and in vitro. The sites of phosphorylation will be determined by tryptic and peptic cleavage of the proteins. To examine the role of phosphorylated BP in the assembly of the myelin membrane, these studies will be extended to determine whether BPs in the brains of Jimpy mice are phosphorylated since this mutant fails to form compact myelin.